lib:util: Avoid free'ing our own pointer

[Samba.git] / docs-xml / Samba-Developers-Guide / modules.xml

<?xml version="1.0" encoding="iso-8859-1"?>
<!DOCTYPE chapter PUBLIC "-//Samba-Team//DTD DocBook V4.2-Based Variant V1.0//EN" "http://www.samba.org/samba/DTD/samba-doc">
<chapter id="modules">
<chapterinfo>
        <author>
                <firstname>Jelmer</firstname><surname>Vernooij</surname>
                <affiliation>
                        <orgname>Samba Team</orgname>
                        <address><email>jelmer@samba.org</email></address>
                </affiliation>
        </author>
        <pubdate> 19 March 2003 </pubdate>
</chapterinfo>

<title>Modules</title>

<sect1>
<title>Advantages</title>

<para>
The new modules system has the following advantages:
</para>

<simplelist>
<member>Transparent loading of static and shared modules (no need
for a subsystem to know about modules)</member>
<member>Simple selection between shared and static modules at configure time</member>
<member>"preload modules" option for increasing performance for stable modules</member>
<member>No nasty #define stuff anymore</member>
<member>All backends are available as plugin now (including pdb_ldap and pdb_tdb)</member>
</simplelist>
</sect1>

<sect1>
<title>Loading modules</title>

<para>
Some subsystems in samba use different backends. These backends can be
either statically linked in to samba or available as a plugin. A subsystem
should have a function that allows a module to register itself. For example,
the passdb subsystem has:
</para>

<para><programlisting>
NTSTATUS smb_register_passdb(int version, const char *name, pdb_init_function init);
</programlisting></para>

<para>
This function will be called by the initialisation function of the module to
register itself.
</para>

<sect2>
<title>Static modules</title>

<para>
The modules system compiles a list of initialisation functions for the
static modules of each subsystem. This is a define. For example,
it is here currently (from <filename>include/config.h</filename>):
</para>

<para><programlisting>
/* Static init functions */
#define static_init_pdb { pdb_mysql_init(); pdb_ldap_init(); pdb_smbpasswd_init(); pdb_tdbsam_init(); pdb_guest_init();}
</programlisting></para>

<para>
These functions should be called before the subsystem is used. That
should be done when the subsystem is initialised or first used.
</para>

</sect2>

<sect2>
<title>Shared modules</title>

<para>
If a subsystem needs a certain backend, it should check if it has
already been registered. If the backend hasn't been registered already,
the subsystem should call smb_probe_module(char *subsystem, char *backend).
This function tries to load the correct module from a certain path
($LIBDIR/subsystem/backend.so). If the first character in 'backend'
is a slash, smb_probe_module() tries to load the module from the
absolute path specified in 'backend'.
</para>

<para>After smb_probe_module() has been executed, the subsystem
should check again if the module has been registered.
</para>

</sect2>
</sect1>

<sect1>
<title>Writing modules</title>

<para>
Each module has an initialisation function. For modules that are
included with samba this name is '<replaceable>subsystem</replaceable>_<replaceable>backend</replaceable>_init'. For external modules (that will never be built-in, but only available as a module) this name is always 'init_module'. (In the case of modules included with samba, the configure system will add a #define subsystem_backend_init() init_module()).
The prototype for these functions is:
</para>

<para><programlisting>
NTSTATUS init_module(TALLOC_CTX *);
</programlisting></para>

<para>This function should call one or more
registration functions. The function should return NT_STATUS_OK on success and
NT_STATUS_UNSUCCESSFUL or a more useful nt error code on failure.</para>

<para>For example, pdb_ldap_init() contains: </para>

<para><programlisting>
NTSTATUS pdb_ldap_init(TALLOC_CTX *)
{
smb_register_passdb(PASSDB_INTERFACE_VERSION, "ldapsam", pdb_init_ldapsam);
smb_register_passdb(PASSDB_INTERFACE_VERSION, "ldapsam_nua", pdb_init_ldapsam_nua);
        return NT_STATUS_OK;
}
</programlisting></para>

<para>
The TALLOC_CTX pointer passed as a parameter must be a long-lived context,
that will stay around for as long as the program that loads the module
exists. It allows the caller to taloc_free any long lived data the
module choses to place on this context on program exit (giving a cleaner
valgrind trace). It should be used by modules in place of talloc_autofree_context(),
use of which makes programs thread-unsafe. Modules that don't care about
free on exist should use the NULL talloc context.
</para>

<sect2>
<title>Static/Shared selection in configure.in</title>

<para>
Some macros in configure.in generate the various defines and substs that
are necessary for the system to work correct. All modules that should
be built by default have to be added to the variable 'default_modules'.
For example, if ldap is found, pdb_ldap is added to this variable.
</para>

<para>
On the bottom of configure.in, SMB_MODULE() should be called
for each module and SMB_SUBSYSTEM() for each subsystem.
</para>

<para>Syntax:</para>

<para><programlisting>
SMB_MODULE(<replaceable>subsystem</replaceable>_<replaceable>backend</replaceable>, <replaceable>object files</replaceable>, <replaceable>plugin name</replaceable>, <replaceable>subsystem name</replaceable>, <replaceable>static_action</replaceable>, <replaceable>shared_action</replaceable>)
SMB_SUBSYSTEM(<replaceable>subsystem</replaceable>,<replaceable>depfile</replaceable>)
</programlisting></para>

<para>The depfile for a certain subsystem is the file that calls the
initialisation functions for the statically built in modules.</para>

<para>
<replaceable>@SUBSYSTEM_MODULES@</replaceable> in Makefile.in will
be replaced with the names of the plugins to build.
</para>

<para>You must make sure all .c files that contain defines that can
be changed by ./configure are rebuilt in the 'modules_clean' make target.
Practically, this means all c files that contain <command>static_init_subsystem;</command> calls need to be rebuilt.
</para>

<note>
<para>
There currently also is a configure.in command called SMB_MODULE_PROVIVES().
This is used for modules that register multiple things. It should not
be used as probing will most likely disappear in the future.</para>
</note>

</sect2>
</sect1>
</chapter>